Planar optical waveguides are gaining increasing acceptance for various applications. In the manufacture of planar optical waveguides, a length of optical fiber, commonly referred to as a "pigtail", is attached to the planar optical waveguide. The pigtail is used during installation of the planar optical waveguide to connect the planar optical waveguide to other components, for example other planar optical waveguides, optical fiber, transmitters or receivers, in a system. Pigtails may be attached to components individually or in blocks containing a plurality of pigtails.
The performance of system components can be degraded by optical reflections at connections, including the connection between the pigtail and the planar optical waveguide. The back reflection from the connection between the pigtail and the planar optical waveguide can result in optical power feedback to components such as lasers and other optical power sources. Although the acceptable level of back reflection depends on the protocols used and the network structure, system designers routinely specify back reflection performance of less than -50 dB for individual components. To reduce the level of back reflection, it is desirable to have a low reflectance connection between the pigtail and the planar optical waveguide.
One method used to reduce the back reflection level is to include optical isolators in the system. However, optical isolators increase optical loss, cost and complexity of the overall system. In addition, optical isolators may introduce wavelength and polarization sensitivities into the system. Also, some recent experiments have shown that, even with the use of optical isolators, severe system degradation can occur in Gbit/second systems due to multiple reflections from connectors. Gimlett et al., "Degradation in Gbit/s DFB Laser Transmission Systems Due to Phase-to-Intensity Noise Conversion by Multiple Reflection Points", Electronics Letters, vol. 24, no. 7, pp. 406-408, 1988.
Bevelling the endface of a fiber is primarily used in fiber-to-fiber connections. A problem with using a bevelled endface of an optical fiber in connecting a pigtail to an optical waveguide component is the subsequent requirement of orientation of the pigtail endface to obtain the best results. One orientation of the pigtail endface is shown in FIG. 2. In particular, without proper orientation, such a connection may be mechanically unstable, especially with respect to certain environmental exposures. This instability is generally caused by movement of the pigtail relative to the component which results from expansion of adhesives or gels used to attach the pigtail to the component, and is affected by the impact of the bevel orientation on adhesive or gel location.
This instability is especially evident in situations where the bevel angle is larger (about 10.degree. or more) and the pigtail endface is parallel to the component endface as shown in FIG. 2. If the endfaces are parallel, the axial motion of the pigtail due to expansion mismatches may force the pigtail endface to move upward such that the optical axis of the pigtail is no longer optically aligned with the optical axis of the component. In addition, the pigtail may be forced upward by a rear adhesive drop (not shown) where the fibers are attached individually. Sufficient adhesive must be present on the upper edge of the pigtail to prevent movement. Limited spacing is allowed between the pigtail core and the component so as to minimize optical losses. Therefore, the bevel orientation affects the instability of the pigtail-to-component connection. For further information regarding the attachment of pigtails to components see, for example, Vial et at. U.S. Pat. No. 5,185,835.
It is also possible to orient a bevelled pigtail endface such that it is in an open configuration with respect to the component, as shown in FIG. 2(b). This open configuration is more stable mechanically, as compared to the parallel configuration of FIG. 2, because of an approximate equilibrium in the volume of adhesive or gel above and below the point at which the pigtail endface is closest to the component and because the translation force is not as great. However, this open configuration increases the pigtail/component coupling loss due to angular beam deviation and is less stable during thermal tests because the index of refraction of the adhesive or gel varies dramatically with temperature, thus increasing losses caused by beam deflection.